Retransmission schemes have been proposed to increase the efficiency of Hybrid Automatic Repeat Request (HARQ) in a MIMO system.
One of the retransmission schemes is basis hopping that artificially provides time diversity to a channel in order to solve the problem of the decrease of HARQ gain on a slow fading channel.
Another retransmission scheme is Trellis-Coded Modulation (TCM) reallocation that reallocates a TCM code to multiple antennas at every retransmission.
A third retransmission scheme is antenna permutation that changes the mapping relation between a transmission stream and an antenna at every retransmission. The second and third retransmission schemes commonly provide time diversity to a slow fading channel.
The other retransmission scheme is to form an Alamouti Space Time Code (STC) by combining retransmission information with initial transmission information.
The first, second and third retransmission schemes aim to provide time diversity in a slow fading channel environment. Therefore, a desired performance improvement cannot be achieved in a fast fading channel environment already having time diversity.
The last retransmission scheme is based on the premise that a retransmission channel is almost constant to form an Alamouti STC with initial transmission information and retransmission information. A desired performance improvement can be achieved only in the slow fading channel environment.
However, in a real wireless communication environment, the channel environment mostly changes at every retransmission because of a time delay involved in signal processing and feedback. Moreover, if a channel is in deep fading status, it may be better to delay retransmission of erroneous information until the deep fading channel status is released. Accordingly, there exists a need for a new retransmission scheme adapted to various real channel environments in order to increase the efficiency of HARQ.